The Proton mail service is owned by the not-for-profit Proton Foundation through its subsidiary Proton AG, which also ...

Sending PGP-encrypted messages between any email service Proton Mail allows you to include the public key needed to decrypt any PGP-encrypted messages you send.

Proton Mail is a user-friendly way of using PGP encryption to turn emails into end-to-end encrypted communication. It only works when both/all parties are using the app.